zephyr/single_loader: make decription working flash

Decryption buffer was fixed to 1024 which imposes failure
while attempting to decrypt bigger image chunks, which is
expected on memories of bigger erase-block-size.

This patch attempt to get teh proper buffer size basing on
SOC's nv flash node.

fixes #1310

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>